Typically, the amorphous steel strip used for such transformer cores is very thin, e.g., only about one mil in thickness as compared to the 7-12 mils typical for grain-oriented silicon steel. One basic approach to making a core from such amorphous steel strip involves unwinding from a spool of strip material an essentially continuous length of strip, cutting from this continuous length sections of appropriate length, and then wrapping these sections about an arbor or the like in appropriate angular positions on the arbor. To make the core assembly process an economical one, it is highly desirable that many essentially continuous strips of the above type, e.g., 10 to 30, be formed into a composite strip and that this composite strip be cut to form sections of multiple-strip (or multiple-layer) thickness that have appropriate lengths. These latter sections can be much more easily and quickly handled than an equivalent number of sections of single-strip (or single-layer) thickness.
One approach for forming the above-described composite strip involves, first, taking spools of strip steel (in the single layer form the strip is received from the mill) and placing such spools on reels (or payoffs) equal in number to the number of thickness layers desired in the composite strip. Then, according to such approach, strip from all these reels is unwound simultaneously, and the unwound portions of such strip are combined in layers to form the desired multiple-layer composite strip.
This approach requires a very large and expensive machine if the composite strip is to have the desired large number of layers. For example, to produce strip of 20 layers would require 20 payoff stands. These payoff stands are expensive, and twenty of them would consume a large amount of floor space as well as being unduly expensive. Another problem with 20 single-strip thickness payoffs is that strips of single-layer thickness are prone to break, and each such break would necessitate stopping the machine. This tendency to break strips would be especially severe in a machine where the feed from the spools is discontinuous, and the strips are thus subjected to repeated decelerating and accelerating forces.
Another approach for making an amorphous steel core involves winding up an annulus of amorphous steel strip derived from a single spool and then cutting the annulus along a radial line to produce a multiplicity of separate strips of appropriate length which fall into a stack. Thereafter, the strips are taken from the stack in groups of multiple-strip thickness and these groups are assembled about an arbor, typically using for this purpose a belt nester that includes a rotating arbor. Examples of this approach are disclosed in our U.S. Pat. No. 4,734,975 and in U.S. Pat. No. 4,741,096-Lee and Ballard, both assigned to the assignee of the present invention.
In this latter approach, as pointed out above, each core is typically made from strip derived from a single spool. While using this approach greatly reduces the number of payoff stands required for the core-making machine, as compared to the earlier-described machine, it is subject to the disadvantage that the space factor of the resulting core is often not as high as might be desired. If the strip material on the starting spool has a low space factor, then the final core will almost always have a low space factor. We have found that by making the core from strip derived from several spools, instead of a single spool, we can produce cores with much more uniform space factors of the desired high level.
For this latter reason, and several others, we utilize for making each core a method that employs strip derived from a plurality of spools. This approach has the additional advantage that we are able to match the strip material present in each core for both physical and electrical characteristics. The manufacturer of the strip typically supplies with each spool data as to the space factor, cross-section, and magnetic properties of the strip in that particular spool. By selecting appropriate combinations of strip, we can more accurately predict the core build and the magnetic losses for the resulting core, and this ability helps us in designing and manufacturing an economical amorphous metal transformer.